- Display mutated requests from request interceptor in curl output in UI.
  Put this behind showMutatedRequest flag so that the mutation can be
  silent.
- Document requestInterceptor, responseInterceptor and showMutatedRequest
  in README.md
- Add tests

I couldn't find any instances of body being an array when testing
formData with real specs... I think this was a mistake.

This reverts commit 666e22f989.

Reopens #3414

This reverts commit 99d368673b4fbd640b15fe85e5c9af52179dc749, reversing
changes made to 8bf9037bbc.

Previously, a definiton with a 'length' property with numeric value
would result in an OrderedMap of that length.

This is now fixed and covered by tests

The request body is an immutable.js OrderedMap rather than a string,
so this fixes the type error:

  TypeError: request.get(...).split is not a function

Adds a special curl command argument for files, e.g.

  curl -F "param=value" -F "file=@filename.txt;type=text/plain"