- Allow the use of system provided certificate setup that may be incorporated
  into the SSL library used on the specific system
  + At present we either use the default certificate bundle we ship with the
    boto source, or we force a user/integrator to create a bundle file of their
    own. Linux distributors build the way certificates are used and validated
    into their SSL implementation. This change allows integrators to use their
    way of certificate handling by setting the configuration to the new
    "system" keyword.
Merge pull request #2139 from rjschwei/systemcerts

Use system supplied certs without a bundle file
Fix dupe header dict
Add failing quoted string test
Corrected quoted string header value preservation
Merge remote-tracking branch 'leetrout/fix/signing-v4-headers' into develop

Fixes #2142.