WL-2194 Encode spaces correctly in HTTP filenames

Spaces would previously be encoded to + which is correct for URLs but not
in this situation. As a result filenames would have a + character where
there should have been a space.

Firefox uses a separate encoding scheme which appears to encode and
decodes spaces correctly.
Merge pull request #23 from ox-it/WL-2194

WL-2194 Encode spaces correctly in HTTP filenames