Clean HTTPPparse in tests

[duboism]

The usage of HTTPParse in class TestBiomajHTTPDownload (and a bit in TestBiomajHTTPSDownload) is messy:

• setUp creates an HTTPParse object (self.http_parse) used in most tests but some tests recreate or modify it. The usage of date_format is weird.
• The value of date_format in the config file contains quotes which seems wrong (and may explain why date_format is set several times for no apparent reason).

This PR cleans the situation:

• The common self.http_parse is created once and not modified and the value of date_format is correct.
• Tests that need a custom HTTPParse create it (using the value in the config file if possible).

We also clean some useless code.

[duboism]

I think that the PR is ready. Any comment ?

[duboism]

@osallou Thanks for merging.