title: Casting string to int null issue
body: <p>I have a spark dataframe, results, that has two string columns I would like to cast to numeric:</p>
  
  <pre><code>&gt;&gt;&gt; results.show()
  +--------------------+-----------------+------------------------+
  |       Hospital Name|HCAHPS Base Score|HCAHPS Consistency Score|
  +--------------------+-----------------+------------------------+
  |"ADIRONDACK MEDIC...|             "43"|                    "20"|
  |"BAYLOR MEDICAL C...|             "32"|                    "20"|
  |"GOOD SHEPHERD ME...|             "25"|                    "20"|
  |"GOOD SHEPHERD ME...|             "25"|                    "20"|
  |"MASONIC HOME AND...|  "Not Available"|         "Not Available"|
  |"ST HELENA HOSPITAL"|             "41"|                    "20"|
  |   "TOURO INFIRMARY"|             "15"|                    "18"|
  |"WAHIAWA GENERAL ...|             "17"|                    "10"|
  |"ANNA JAQUES HOSP...|             "27"|                    "18"|
  |    "CMC-BLUE RIDGE"|             "31"|                    "18"|
  |"EVANSTON REGIONA...|             "15"|                    "15"|
  |"OKLAHOMA SPINE H...|             "79"|                    "20"|
  |"PICKENS COUNTY M...|  "Not Available"|         "Not Available"|
  |"PORTNEUF MEDICAL...|             "11"|                    "17"|
  |"PRESENCE SAINT J...|             "20"|                    "17"|
  |"RIVERSIDE MEDICA...|             "39"|                    "20"|
  |"RIVERSIDE MEDICA...|             "39"|                    "20"|
  |"RIVERSIDE MEDICA...|             "39"|                    "20"|
  |"SOUTH GEORGIA ME...|    "3 out of 10"|                    "24"|
  |"TAMPA GENERAL HO...|             "23"|                    "16"|
  +--------------------+-----------------+------------------------+
  </code></pre>
  
  <p>Attempting such gives me a table of null values:</p>
  
  <pre><code>&gt;&gt;&gt; results2 = results.select( results["Hospital Name"], results["HCAHPS Base Score"].cast(pe()).alias("HCAHPS Base Score"), results["HCAHPS Consistency Score"].cast(IntegerType()).aHPS Consistency Score") )
  &gt;&gt;&gt; results2.show()
  +--------------------+-----------------+------------------------+
  |       Hospital Name|HCAHPS Base Score|HCAHPS Consistency Score|
  +--------------------+-----------------+------------------------+
  |"ADIRONDACK MEDIC...|             null|                    null|
  |"BAYLOR MEDICAL C...|             null|                    null|
  |"GOOD SHEPHERD ME...|             null|                    null|
  |"GOOD SHEPHERD ME...|             null|                    null|
  |"MASONIC HOME AND...|             null|                    null|
  |"ST HELENA HOSPITAL"|             null|                    null|
  |   "TOURO INFIRMARY"|             null|                    null|
  |"WAHIAWA GENERAL ...|             null|                    null|
  |"ANNA JAQUES HOSP...|             null|                    null|
  |    "CMC-BLUE RIDGE"|             null|                    null|
  |"EVANSTON REGIONA...|             null|                    null|
  |"OKLAHOMA SPINE H...|             null|                    null|
  |"PICKENS COUNTY M...|             null|                    null|
  |"PORTNEUF MEDICAL...|             null|                    null|
  |"PRESENCE SAINT J...|             null|                    null|
  |"RIVERSIDE MEDICA...|             null|                    null|
  |"RIVERSIDE MEDICA...|             null|                    null|
  |"RIVERSIDE MEDICA...|             null|                    null|
  |"SOUTH GEORGIA ME...|             null|                    null|
  |"TAMPA GENERAL HO...|             null|                    null|
  +--------------------+-----------------+------------------------+
  
  only showing top 20 rows
  </code></pre>
  
  <p>Is it not possible to cast string columns to integer in pyspark?</p>