Image assets in css files do not get an MD5 hash when they contain query strings or hashes

I have encountered a bug in connect-assets where image assets (or in my case font assets) do not get an MD5 hash when they contain a query string or hash. I have fixed this bug. I believe I have captured all the cases in the tests, but please have a look over and see if you can think of any other edge cases we should look out for.

FYI the production CSS that I am having the issue on is this:
```css
@font-face {
  font-family: 'SourceSansProRegular';
  src: url('/content/fonts/SourceSansPro-Regular-webfont.eot');
  src: url('/content/fonts/SourceSansPro-Regular-webfont.eot?#iefix') format('embedded-opentype'), url('/content/fonts/SourceSansPro-Regular-webfont.woff') format('woff'), url('/content/fonts/SourceSansPro-Regular-webfont.ttf') format('truetype'),  url('/content/fonts/SourceSansPro-Regular-webfont.svg#SourceSansProRegular') format('svg');
  font-weight: normal;
  font-style: normal;
}
```

Which gets generated in production as this (resulting in 404 not found on the server):
```css
@font-face {
font-family:'SourceSansProRegular';
src:url(/content/fonts/SourceSansPro-Regular-webfont-e9e65e67001fc8ce81ab9345c676a085.eot);
src:url(/content/fonts/SourceSansPro-Regular-webfont.eot?#iefix) format("embedded-opentype)","url(/content/fonts/SourceSansPro-Regular-webfont-608b244358954d2f861c952fa8927d0c.woff") format("woff)","url(/content/fonts/SourceSansPro-Regular-webfont-2bd8bac925bf9e744adfc97019035b8c.ttf") format("truetype)","url(/content/fonts/SourceSansPro-Regular-webfont.svg#SourceSansProRegular") format("svg");
font-weight:400;
font-style:normal
}
```
But I believe should be generated as this:
```css
@font-face {
font-family:'SourceSansProRegular';
src:url(/content/fonts/SourceSansPro-Regular-webfont-e9e65e67001fc8ce81ab9345c676a085.eot);
src:url(/content/fonts/SourceSansPro-Regular-webfont-e9e65e67001fc8ce81ab9345c676a085.eot#iefix) format("embedded-opentype)","url(/content/fonts/SourceSansPro-Regular-webfont-608b244358954d2f861c952fa8927d0c.woff") format("woff)","url(/content/fonts/SourceSansPro-Regular-webfont-2bd8bac925bf9e744adfc97019035b8c.ttf") format("truetype)","url(/content/fonts/SourceSansPro-Regular-webfont-a2454401f56e0619a2e270dfa24fdb5d.svg#SourceSansProRegular") format("svg");
font-weight:400;
font-style:normal
}
```