Improper sorting order for some cases

[ngseer]

• aiohttp JSON API version: 0.37.0
• Python version: 3.7.0
• Operating System: ArchLinux (w/ 4.18.8 kernel)

Description

In some rare cases JSONAPIContext doesn't follow JSONAPI specs about sorting order.

It happens when a field with - prefix is followed by a field without prefix (see example below).

What I Did

In [1]: from aiohttp_json_api.context import JSONAPIContext

In [2]: class Request:
   ...:     # Imitate aiohttp request
   ...:     query = {'sort': '-field1,field2,+field3'}
   ...:     

In [3]: JSONAPIContext.parse_request_sorting(Request())
Out[3]: 
OrderedDict([(('field1',), <SortDirection.DESC: '-'>),
             (('field2',), <SortDirection.DESC: '-'>),
             (('field3',), <SortDirection.ASC: '+'>)])

In according with the specs, field2 must have an ascending order.

It seems that this misbehaviour is caused by these lines and direction variable should be initialize inside the loop.