Improve tests coverage

[mastilver]

Those are not covered by tests:

• /~https://github.com/danethurber/webpack-manifest-plugin/blob/master/lib/plugin.js#L34-L37 (add test: outputs a manifest of no-js file #59)
• /~https://github.com/danethurber/webpack-manifest-plugin/blob/master/lib/plugin.js#L52-L54
• /~https://github.com/danethurber/webpack-manifest-plugin/blob/master/lib/plugin.js#L58 (add test: outputs a manifest of no-js file #59)
• /~https://github.com/danethurber/webpack-manifest-plugin/blob/master/lib/plugin.js#L68-L70 (add test: outputs a manifest of no-js file #59)
• /~https://github.com/danethurber/webpack-manifest-plugin/blob/master/lib/plugin.js#L108-L113 (fix: make sure manifest writes don't overlap #56)

[a-x-]

I'm starting look at #L34-L37

[mastilver]

I'm working on L52-L54
WIP: /~https://github.com/danethurber/webpack-manifest-plugin/tree/test-update-hot