Edited Issue: ReturnType of an action or function should be an element in $metadata document. [1572]

ReturnType of an action or function should be an element in $metadata document according to the CSDL v4 protocol, but now it is an attribute.

Repro steps:
1. Open Fiddler
2. Run E2E test case ProvideOverloadToSupplyEntitySetConfiguration in ActionMetadataTests.cs
3. View the traffic in Fiddler

Expected:

<Action Name="GetProductsByCategory" IsBound="true" Nullable="false" m:IsAlwaysBindable="true">
<ReturnType Type="Collection(WebStack.QA.Test.OData.ModelBuilder.ActionProduct)" />
<Parameter Name="bindingParameter" Type="WebStack.QA.Test.OData.ModelBuilder.ActionProduct" />
</Action>
Actual:
<Action Name="GetProductsByCategory" IsBound="true" ReturnType="Collection(WebStack.QA.Test.OData.ModelBuilder.ActionProduct)" Nullable="false" m:IsAlwaysBindable="true">
<Parameter Name="bindingParameter" Type="WebStack.QA.Test.OData.ModelBuilder.ActionProduct" />
</Action>