Fixed --browser behaviour when --server-name provided

[marc1n]

Fixing PR #1533: Calling:

   pserve --server-name abc --browser app.ini

caused error : "LookupError: No section 'main' (prefixed by 'server') found in config app.ini"

[digitalresistor]

Please provide fix against master branch, we will back port as necessary to the various branches, this way your fix won't get lost on a single branch.

[mmerickel]

1. Please add a commit with your name in CONTRIBUTORS.txt.
2. Please at least submit a PR for master as @bertjwregeer.
3. We will usually backport the fix as necessary but you're free to also submit a backport to help us out if you like!