title: pandas merge from mysql datatype mismatch issue
body: <p>Consider the following:</p>
  
  <pre><code>import pandas as pd
  
  df1 = pd.io.sql.read_frame("select c1,c2,c3 from table1;",mysqldb1)
  df2 = pd.io.sql.read_frame("select c1,c4 from table1;",mysqldb2)
  df =  pd.merge(df1,df2,on='c1',how='inner')
  </code></pre>
  
  <p>This works most of the time.  I believe I am having a problem because in mysqldb1 c1 is a varchar whereas in mysqldb2 c1 is an INTEGER.  I can't change this state of affairs.  But now, because </p>
  
  <pre><code>df1.c1.dtype != df2.c1.dtype
  </code></pre>
  
  <p>pd.merge returns an empty dataframe.</p>
  
  <p>I have tried casting, i.e. </p>
  
  <pre><code>df2['c1'] = df2.c1.astype(object) 
  </code></pre>
  
  <p>and then tried the merge, to no avail.</p>