'Invalid format' error about pygeoapi

[justb4]

To be complete some tests need to be added in test_check_format() for "browser Accept Header" where we initially ran into and was the reason for this issue. A typical browser Accept Header may look like:

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3

A PR is a bit overkill to add lines in test_check_format() :

headers_['Accept'] = 'text/html,application/xhtml+xml,application/xml;q=0.9,'
assert check_format({}, headers_) == 'html'

headers_['accept'] = 'text/html,application/xhtml+xml,application/xml;q=0.9,'
assert check_format({}, headers_) == 'html'

[tomkralidis]

Thanks @justb4; added in 20c96a8