Fix for httpclient 2.5.3+

Checkout:
https://github.com/nahi/httpclient/compare/v2.5.2...v2.5.3#diff-9c128a1e92647c91c6efad47c0709bf7L198

In 2.5.2, it's taking the `uri`, but in 2.5.3, it's taking the `req`.
We could also detect by versions, but I chose to detect by if
`default_header` is defined here:
https://github.com/nahi/httpclient/compare/v2.5.2...v2.5.3#diff-2b61e1f08102d46865dd580cfa9bc88aR329

Either way should be fine, no strong opinions here.
(Detecting by method signature would be the best, but the arity
 is the same, and I am not sure how else we could detect)
Merge pull request #430 from godfat/adopt-httpclient-2.5.3

Fix for httpclient 2.5.3+