Apply default style for not overwritten properties

[Jesus89]

Default properties (color, width, strokeColor, strokeWidth) should be applied always unless they are overwritten by the user. For example: String API 'color: blue' should overwrite the color, but keep the rest of the properties. The same for the Dict API { 'color: 'blue' }.

The implementation is easy for the Dict API, just update the defaults dictionary with the style one. For the String API it's a bit more tricky because we need to detect the style properties contained in the string ('color:' in style) to remove the defaults, and then serialize the rest of defaults.

[Jesus89]

Closed via #685