only pass alpha value to fill colour if it is non-1

[thomas-mcdonald]

this enables better performance by the optimiser - if an alpha value is present SVGO will not attempt to minimise it.

in the best case, this enables rgba(255,255,255,1) -> #fff

fixes #9

[coveralls]

Pull Request Test Coverage Report for Build 21

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 91.139%

---

Totals
Change from base Build 20:	0.5%
Covered Lines:	56
Relevant Lines:	59

---

💛 - Coveralls

[haithembelhaj]

Great thanks!!!