fix(karma): set defaults for karma.conf.js

[JSMike]

The karma plugin is currently overwriting configuration set in karma.conf.js instead of merging existing values. This fix will set defaults but not overwrite existing configuration.

[JSMike]

Note, I ran into this issue when trying to add the following to the karma.conf.js:

    webpackMiddleware: {
      watchOptions: {
        poll: 1000
      }
    }

With the current karma plugin all webpack and webpackMiddleware configuration in karma.conf.js is overwritten.

With this fix the defaults are set for webpack and webpackMiddleware, and any values set in karma.conf.js will also be applied, and/or overwrite the defaults.

[JSMike]

Cleaned up PR to create variable instead of using inline object.

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