Fix stoke color and non-stroke color in PDFGraphicState

[tongbaojia]

Dear Pdfminer maintainers.

I ran into this issue finding that the color coding for the curve is not saved.
This is traced down to a potential bug.
.color is not an attribute of graphicstate, while scolor and ncolor are.
I think this was a typo from before, and the issue was fixed in do_G and do_g but not in do_RG.

[pietermarsman]

I think you are right.

In e6ad15a (in PR #37 by @begnini) the .scolor (stroking color) and .ncolor (non-stroking color) where introduced. However, only .do_SCN used these attributes. Six other functions (.do_G(), .do_g(), .do_RG(), .do_rg(), .do_K(), .do_k()) use the .color attribute, which I find very perculiar.

In 2231f08 (in PR #129 by @pquentin) 2 methods where fixed: .do_G() and .do_g(), the others remained the same.

[keshavbohra]

Can anyone change the build for pdfminer.six in PyPI with the above changes.

[pietermarsman]

This should be merged before releasing

[tataganesh]

All PR's should be branched out of "develop" and merged into develop if possible.