Add headers arg to GraphQLTestCase.query

[chibie]

Closes #817, closes #824

Now headers can be passed to graphql URL

For example, authorization header can be set thus:

...
response = self.query(
    '''
    mutation updateUser($input: UserUpdateInput!) {
        updateUser(input: $input) {
            ok
            errors {
                field
                messages
            }
        }
    }
    ''',
    op_name='updateUser',
    input_data={
        'firstName': 'Dumebi'
    },
    headers={'HTTP_AUTHORIZATION': f'JWT {token}'}
)

[jkimbo]

This looks good thanks @chibie !

[AliMirjahani]

Why should we use HTTP_AUTHORIZATION instead of Authorization?

[everdrone]

Second this, the HTTP_AUTHORIZATION is undocumented as far as i know.
Why use it instead of Authorization @chibie?