Fix decoding for single frame GIF

[onevcat]

Fix for #231

[codecov-io]

Current coverage is 75.55%

Merging #234 into master will increase coverage by +0.03% as of 3592e84

@@            master    #234   diff @@
======================================
  Files           11      11       
  Stmts         1250    1252     +2
  Branches         0       0       
  Methods          0       0       
======================================
+ Hit            944     946     +2
  Partial          0       0       
  Missed         306     306       

Review entire Coverage Diff as of 3592e84

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[cache0928]

新版本下貌似某些jpg也会出现下载问题，例如图片：http://ww2.sinaimg.cn/bmiddle/66966373jw1f108cxxuw4j20bl0rswh8.jpg

[onevcat]

@cache0928 这张图我这边没有问题。