title: Support v7 GridLayout doesn't stretch columns
body: <p>I'm trying to use GridLayout from support library in my project. All that I've done was adding it to my project as Android library. I've created a simple layout to show what is not workning.</p>
  
  <pre><code>&lt;ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:tools="http://schemas.android.com/tools"
  xmlns:app="http://schemas.android.com/apk/res/com.example.myapp"
  android:layout_width="match_parent"
  android:layout_height="match_parent" &gt;
  
  &lt;android.support.v7.widget.GridLayout
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:padding="@dimen/margin"
      app:columnCount="3"
      app:rowCount="8"
      tools:context=".RegistrationActivity" &gt;
  
      &lt;Button
          android:id="@+id/activity_registration_company"
          app:layout_columnSpan="3"
          android:background="@android:color/darker_gray"
          android:text="1" /&gt;
  
      &lt;Button
          android:id="@+id/activity_registration_street"
          app:layout_columnSpan="2"
          android:background="@android:color/darker_gray"
          android:text="2" /&gt;
  
      &lt;Button
          android:id="@+id/activity_registration_building"
          app:layout_columnSpan="1"
          android:background="@android:color/darker_gray"
          android:text="3" /&gt;
  &lt;/android.support.v7.widget.GridLayout&gt;
  </code></pre>
  
  <p></p>
  
  <p>And here is what I see in Eclipse layout editor (the same on device):
  <img src="https://i.stack.imgur.com/9UrCF.png" alt="enter image description here"></p>
  
  <p>As you can see GridLayout's content doesn't occupy the whole width. Also Button doesn't occupy the space defined by spans. Any ideas?</p>