Make user-supplied resizerClassName prop additive

[dfsp-spirit]

If a resizerClassName prop is supplied by the user, make it additive to the default class name (instead of completely replacing it).

[coveralls]

Coverage increased (+0.09%) to 90.686% when pulling b9815b1 on dfsp-spirit:make_resizerClassName_prop_additive_to_Resizer_defaultProps into 7c88eec on tomkp:master.

[coveralls]

Coverage increased (+0.09%) to 90.686% when pulling fe42b66 on dfsp-spirit:make_resizerClassName_prop_additive_to_Resizer_defaultProps into 7c88eec on tomkp:master.

[dfsp-spirit]

@tomkp This PR fixes that if you do something like

`<SplitPane split="horizontal" minSize={50} defaultSize={50} resizerClassName="my_custom_css_class">`

the resulting Resizer element does not have the "Resizer" class name anymore and thus all the CSS is not applied to it, effectively breaking it.

Without this PR, you have to do this to get it to work:

 `<SplitPane split="horizontal" minSize={50} defaultSize={50} resizerClassName="Resizer my_custom_css_class">`

I am not sure what your intended behavior is, but I would assume the resizerClassName prop to be additive.

[coveralls]

Coverage increased (+0.09%) to 90.686% when pulling 7dbfc1e on dfsp-spirit:make_resizerClassName_prop_additive_to_Resizer_defaultProps into 7c88eec on tomkp:master.