Add support for sending POST with multipart/form-data

[datagutten]

Some sites require POST requests to be sent as multipart/form-data.
When passing an array as POST-data cURL defaults to multipart/form-data, but Requests will flatten and urlencode it. To avoid this I check if a Content-Type header containing multipart/form-data is set in the arguments to Requests::post.
I have implemented this for both cURL and fsockopen.

[codecov-io]

Codecov Report

Merging #371 into master will decrease coverage by 0.12%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #371      +/-   ##
============================================
- Coverage     92.11%   91.98%   -0.13%     
- Complexity      760      766       +6     
============================================
  Files            21       21              
  Lines          1762     1772      +10     
============================================
+ Hits           1623     1630       +7     
- Misses          139      142       +3

Impacted Files	Coverage Δ	Complexity Δ
library/Requests/Transport/cURL.php	93.77% <100%> (+0.09%)	69 <0> (+3)	⬆️
library/Requests/Transport/fsockopen.php	94.41% <100%> (+0.22%)	71 <0> (+3)	⬆️
library/Requests.php	74.57% <0%> (-1.02%)	118% <0%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a009398...99e8e86. Read the comment docs.