Fixing the bug with at-rule

This PR fixes the bug with no whitespace between at-rule without params and the following brace: `@font-face{`. The error was:

``` sh
TypeError: Cannot call method 'replace' of undefined
    at wringAtRule (/Users/kizu/Projects/node-csswring/lib/csswring.js:291:19)
    at /Users/kizu/Projects/node-csswring/node_modules/postcss/lib/container.js:374:30
    at /Users/kizu/Projects/node-csswring/node_modules/postcss/lib/container.js:110:26
    at Root.Container.each (/Users/kizu/Projects/node-csswring/node_modules/postcss/lib/container.js:88:20)
    at Root.Container.eachInside (/Users/kizu/Projects/node-csswring/node_modules/postcss/lib/container.js:109:21)
    at Root.Container.WithRules.constructor$0.eachAtRule (/Users/kizu/Projects/node-csswring/node_modules/postcss/lib/container.js:372:21)
    at CSSWring.postcss (/Users/kizu/Projects/node-csswring/lib/csswring.js:334:7)
    at PostCSS.process (/Users/kizu/Projects/node-csswring/node_modules/postcss/lib/postcss.js:35:28)
    at CSSWring.wring (/Users/kizu/Projects/node-csswring/lib/csswring.js:340:38)
    at Function.csswring.wring (/Users/kizu/Projects/node-csswring/lib/csswring.js:358:25)
```