feat(client): add rawRequest to callback arguments

[matt-forster]

Use case: Allows clients who send requests asynchronously to accurately log the requests they are making.

Adds the XML raw request as an argument to the client callback. This allows correct tracking of the requests, as requests made asynchronously on the same client will overwrite the client.lastRequest attribute.

I added a test to the headers describe block as that is where it seemed similar code was being tested.

[coveralls]

Coverage remained the same at 93.648% when pulling f4f025d on autovance:master into f8f5aef on vpulim:master.

[coveralls]

Coverage remained the same at 93.648% when pulling 242f667 on autovance:master into f8f5aef on vpulim:master.

[jsdevel]

@herom what are your thoughs?

[herom]

@jsdevel @forstermatth This seems like a really useful addition to our library - thanks a lot 👍