Use actual indentation length when setting indentation for row.

Closes #1293

spec/editor-spec.coffee

@@ -2722,6 +2722,15 @@ describe "Editor", ->
         editor.moveCursorLeft()
         expect(editor.getCursorBufferPosition()).toEqual [0, 0]
 
+    describe ".setIndentationForBufferRow", ->
+      describe "when the editor uses soft tabs but the row has hard tabs", ->
+        it "only replaces whitespace charachters", ->
+          editor.setSoftWrap(true)
+          editor.setText("	1\n	2")
+          editor.setCursorBufferPosition([0, 0])
+          editor.setIndentationForBufferRow(0, 2)
+          expect(editor.getText()).toBe("    1\n	2")
+
   describe "when the editor's grammar has an injection selector", ->
     beforeEach ->
       atom.packages.activatePackage('language-text', sync: true)

src/editor.coffee

@@ -269,10 +269,9 @@ class Editor extends Model
   # * newLevel:
   #   A {Number} indicating the new indentation level.
   setIndentationForBufferRow: (bufferRow, newLevel) ->
-    currentLevel = @indentationForBufferRow(bufferRow)
-    currentIndentString = @buildIndentString(currentLevel)
+    currentIndentLength = @lineForBufferRow(bufferRow).match(/^\s*/)[0].length
     newIndentString = @buildIndentString(newLevel)
-    @buffer.change([[bufferRow, 0], [bufferRow, currentIndentString.length]], newIndentString)
+    @buffer.change([[bufferRow, 0], [bufferRow, currentIndentLength]], newIndentString)
 
   # Public: Returns the indentation level of the given line of text.
   #