fix(fixRequestBody): support text/plain

[knudtty]

Description

Adds Content-Type text/plain to bodies that are forwarded w/ fixRequestBody. Crucial since bodyParser handles that content-type.

Motivation and Context

Discovered this issue when a proxy server that was using bodyParser used fixRequestBody to forward bodies over to an API. The API would receive a request with a non-zero Content-Length but an empty body, so the API would be in deadlock trying to read from a stream that never would never have any data.

fixes 1102

How has this been tested?

I added a unit test for this case

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[pkg-pr-new]

npm i https://pkg.pr.new/http-proxy-middleware@1103

commit: f58e5ad

[coveralls]

coverage: 97.129% (+0.01%) from 97.115%
when pulling f58e5ad on knudtty:knudtty/text-plain-fix-request-body
into cf72bcd on chimurai:master.

[chimurai]

Thanks for this PR 🙏