Fixes fa-stack alignment

Added failing test case
Closes #2213

css/font-awesome.css

@@ -195,6 +195,7 @@
 .fa-stack-1x,
 .fa-stack-2x {
   position: absolute;
+  left: 0;
   width: 100%;
   text-align: center;
 }

css/font-awesome.min.css

@@ -19,7 +19,7 @@
 .fa-flip-horizontal{filter:progid:DXImageTransform.Microsoft.BasicImage(rotation=0, mirror=1);-webkit-transform:scale(-1, 1);-moz-transform:scale(-1, 1);-ms-transform:scale(-1, 1);-o-transform:scale(-1, 1);transform:scale(-1, 1)}
 .fa-flip-vertical{filter:progid:DXImageTransform.Microsoft.BasicImage(rotation=2, mirror=1);-webkit-transform:scale(1, -1);-moz-transform:scale(1, -1);-ms-transform:scale(1, -1);-o-transform:scale(1, -1);transform:scale(1, -1)}
 .fa-stack{position:relative;display:inline-block;width:2em;height:2em;line-height:2em;vertical-align:middle}
-.fa-stack-1x,.fa-stack-2x{position:absolute;width:100%;text-align:center}
+.fa-stack-1x,.fa-stack-2x{position:absolute;left:0;width:100%;text-align:center}
 .fa-stack-1x{line-height:inherit}
 .fa-stack-2x{font-size:2em}
 .fa-inverse{color:#fff}

less/stacked.less

@@ -11,6 +11,7 @@
 }
 .@{fa-css-prefix}-stack-1x, .@{fa-css-prefix}-stack-2x {
   position: absolute;
+  left: 0;
   width: 100%;
   text-align: center;
 }

scss/_stacked.scss

@@ -11,6 +11,7 @@
 }
 .#{$fa-css-prefix}-stack-1x, .#{$fa-css-prefix}-stack-2x {
   position: absolute;
+  left: 0;
   width: 100%;
   text-align: center;
 }

src/_includes/tests/stacked.html

@@ -1,3 +1,10 @@
+<span style="border: solid 1px #d3d3d3; display: inline-block; text-align: center">
+  <span class="fa-stack">
+    <i class="fa fa-square fa-stack-2x"></i>
+    <i class="fa fa-twitter fa-stack-1x fa-inverse"></i>
+  </span><br>Center
+</span>
+<hr>
 <p>
   <span class="fa-stack">
     <i class="fa fa-square fa-stack-2x"></i>

src/assets/font-awesome/less/stacked.less

@@ -14,6 +14,7 @@
 }
 .@{fa-css-prefix}-stack-1x, .@{fa-css-prefix}-stack-2x {
   position: absolute;
+  left: 0;
   width: 100%;
   text-align: center;
 }

src/assets/font-awesome/scss/_stacked.scss

@@ -14,6 +14,7 @@
 }
 .#{$fa-css-prefix}-stack-1x, .#{$fa-css-prefix}-stack-2x {
   position: absolute;
+  left: 0;
   width: 100%;
   text-align: center;
 }