(Realted to flot issue flot#1031: flot#1031)

Currently, if the user declares a custom color palette with less than 5 colors (say, n), $.extend(true, options, opts) only modifies the first n colors of the default palette and leaves the last 5-n in place. When the number of series is >n, colors are used that are not part of user-defined palette, contrary to description of colors array in API.

This line overrides the extended colors array and replaces it with exactly the user-defined colors array, when present. Afterwards, the user color palette is respected by the auto tinting/shading system for when there are more series than colors.

Per request from dnschnur (flot#1034 (comment)), I have: 
- Added a comment explaining the purpose of the new check/override
- Changed the structure to match preferred standard structure