Replace space to   for HTML Rendering.

Reason:
Some javascript frameworks(ex. vue.js) manipulate HTML tags when drawing.

At that time, if there is a space-only DOM element, the space in the DOM
element is deleted.

Before:
`` `
<span> </span>
`` `

After (space deleted)
`` `
<span></span>
`` `

In order to avoid this, it has been modified to render with &nbsp; only when there is no diff.

src/line-by-line-printer.js

@@ -211,6 +211,10 @@
       lineWithoutPrefix = lineWithPrefix.line;
     }
 
+    if (prefix === ' ') {
+      prefix = ' ';
+    }
+
     return hoganUtils.render(genericTemplatesPath, 'line',
       {
         type: type,

src/side-by-side-printer.js

@@ -244,6 +244,10 @@
       lineWithoutPrefix = lineWithPrefix.line;
     }
 
+    if (prefix === ' ') {
+      prefix = ' ';
+    }
+
     return hoganUtils.render(genericTemplatesPath, 'line',
       {
         type: type,