Add Net::HTTP.put method

[Zopolis4]

As per title, as well as adding some missing documentation about Net::HTTP#put (and also adding documentation about the new Net::HTTP.put method)

Sanity tests pass locally:

Running rake test failed for me, but that seemed to be a limitation of webrick?

Not sure, which is why I am marking this PR as draft until I can find out where the fault lies.

[Zopolis4]

Looked into it further-- webrick, at least in the way it is used here, does not appear to support PUT.

This became evident when creating the function _test_request__PUT (akin to _test_request__POST), which failed despite using Net::HTTP::Put.new.

[hsbt]

Seems useful. Thanks.