Enable online tests and Proxy adapter tests + various fixes covered by these tests

[michalbundyra]

• Are you fixing a bug?
  • Detail how the bug is invoked currently.
  • Detail the original, incorrect behavior.
  • Detail the new, expected behavior.
  • Base your feature on the master branch, and submit against that branch.
  • Add a regression test that demonstrates the bug, and proves the fix.
  • Add a CHANGELOG.md entry for the fix.

This PR enables online tests and tests for adapters which were before skipped.
Enabling tests shows multiple various errors in different part of the library:

• wrong merging options in Curl adapter - constants has int values, so it was not possible to override the same option
• type of parameter is not checked in Proxy::setOptions
• exception was thrown when empty content has been written through Proxy adapter

Travis CI is using apache with configured forward proxy so all tests can now run.