scrapy.FormRequest.from_response observed to eliminate duplicate keys in formdata

[starrify]

This looks good:

In [2]: scrapy.FormRequest('http://example.com', method='GET', formdata=(('foo', 'bar'), ('foo', 'baz')))
Out[2]: <GET http://example.com?foo=bar&foo=baz>

While here is the issue:

In [3]: response = scrapy.http.TextResponse(url='http://example.com', body='<form></form>', encoding='utf8')

In [4]: scrapy.FormRequest.from_response(response, method='GET', formdata=(('foo', 'bar'), ('foo', 'baz')))
Out[4]: <GET http://example.com?foo=baz>

(Tested with Scrapy 1.5.0 and Python 3.6.5)

[starrify]

Related code:

scrapy/scrapy/http/request/form.py

Line 117 in dd06441

formdata = dict(formdata or ())

[CCInCharge]

I'll take a look if no one's jumped on it yet.