Fix/move charset top

[maxledoux]

@charset must be on the first line of the concatenated file. However, the current code checks if @charset is already at position 0 (at the very beginning of the file). Only if @charset is already at the beginning will the code then attempt to move it to the beginning (where it already is located).

This PR changes if ( 0 === strpos( $buf, '@charset' ) ) { to false !== strpos( $buf, '@charset' ) so that it checks if @charset is present anywhere in $buf. Then it deletes the instance of @charset from $buf before the existing code adds the @charset to the beginning of $buf. Otherwise, we end up with multiple @charset declarations in the concatenated file. There may be better ways to avoid this.

The following is an example of how I tested this:

style1.css:

body  {
	font-size: 12px;
}

style2.css:

.span {
	color: #fff;
}
@charset "UTF-8";
.class {
	color: #000;
}

With the current code the code the concatenated file looks like:

body{font-size: 12px}@charset "UTF-8";.span {color: #fff}.class {color: #000}

Notice the current code does move @charset, but to the top of the contents of the original file, not the top of the concatenated file.

With the commits in this PR the concatenated file would like like:

@charset "UTF-8";
body{font-size: 12px}.span {color: #fff}.class {color: #000}

With my first commit but without my second commit (86331aa) it looked like:

@charset "UTF-8";
body{font-size: 12px}@charset "UTF-8";.span {color: #fff}.class {color: #000}

Hope this is helpful!