Keep the same scheme (protocol) as was used in Jenkins constructor in all requests

Some Jenkins instances are configured to allow reading the data over HTTP scheme, while for manipulating with
data (e.g. create or delete job) HTTPS scheme is required. Allow user to specify preferred URL scheme (by
specifying Jenkins URL) and use this scheme also manipulations with jobs.

This was introduced in 4f29f6742410736016b568002d8569f19d786841 (version 0.1.13) but lost again in
79934f0b5ca44a1d4ecbaf534eb1add8e343193f.
Add kerberos authentication requester using requests_kerberos
Merge pull request #157 from pkocandr/unstable-krbrequester

Add kerberos authentication requester using requests_kerberos