requrests 2.11 requires string only headers

[tetor]

api.projects.get_by_slug() can't work because requests 2.11 changed to accept string only headers.

https://github.com/kennethreitz/requests/issues/3477

Error

Traceback (most recent call last):
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/utils.py", line 796, in check_header_validity
    if not pat.match(value):
TypeError: expected string or buffer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/taiga/requestmaker.py", line 102, in get
    params=query
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/api.py", line 70, in get
    return request('get', url, params=params, **kwargs)
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/api.py", line 56, in request
    return session.request(method=method, url=url, **kwargs)
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/sessions.py", line 461, in request
    prep = self.prepare_request(req)
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/sessions.py", line 394, in prepare_request
    hooks=merge_hooks(request.hooks, self.hooks),
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/models.py", line 295, in prepare
    self.prepare_headers(headers)
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/models.py", line 409, in prepare_headers
    check_header_validity(header)
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/requests/utils.py", line 800, in check_header_validity
    "not %s" % (value, type(value)))
requests.exceptions.InvalidHeader: Header value True must be of type str or bytes, not <class 'bool'>

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "src/taiga_io.py", line 28, in <module>
    pprint(api.me())
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/taiga/client.py", line 88, in me
    return self.users.get('me')
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/taiga/models/base.py", line 47, in get
    endpoint=self.instance.endpoint, id=resource_id
  File "/Users/tetor/.pyenv/versions/cf-tpp/lib/python3.4/site-packages/taiga/requestmaker.py", line 109, in get
    'Network error!', 'GET'
taiga.exceptions.TaigaRestException: Network error!

[yakky]

Thanks @tetor for reporting
Fixed in #43
Released in 0.8.6