Summary:
Network Plugin - When creating a mock request from a selected request, the request data is not in the proper format. It is decoded instead of just being copied from the call (which has already been decoded properly). This PR fixes that problem.
Below is a screenshot showing the problem (which occurs for all text response data):

## Changelog
Network Plugin - Fix problem with decoding request data for mocks copied from selection
Pull Request resolved: https://github.com/facebook/flipper/pull/2340
Test Plan:
Using the sample Android app, issue a network request
In Flipper, create a mock for the network request by selecting it and using the "Copy Selected Calls" function in the mock
Verify that the request data is readable:

Note:
Testing was done using the sample app which uses responses with JSON data. I was not able to provide testing for other types of calls, specifically calls that would return binary data.
Reviewed By: passy
Differential Revision: D28533224
Pulled By: mweststrate
fbshipit-source-id: ce11d23ade60843c110286f7a5bbeba91febbcf0