RequestMappingTemplateS3Location does not work with Fn::If

I have a template for an AppSync Pipeline Function that references a different template depending on a condition. While SAM normally translates the location to the proper s3 location, it does not work if I have an Fn::If.

I would have expected that SAM would translate both locations specified in the Fn::If.

Example:

```yaml
EnvironmentPipelineFunc:
Type: "AWS::AppSync::FunctionConfiguration"
Condition: IsProd
Properties:
Name: "Environment"
RequestMappingTemplateS3Location: !If [IsProd, "prodRequest.vm", "testRequest.vm"]
ResponseMappingTemplateS3Location: !If [IsProd, "prodResponse.vm", "testResponse.vm"]
DataSourceName: !GetAtt "VTLDataSource.Name"
FunctionVersion: "2018-05-29"
ApiId: !GetAtt "API.ApiId"
```