fixed class concatenation

gluon/sqlhtml.py

@@ -840,7 +840,7 @@ def formstyle_bootstrap(form, fields):
             controls.add_class('span4')
 
         if isinstance(label, LABEL):
-            label['_class'] += ' control-label'
+            label['_class'] = add_class(label.get('_class'),'control-label')
 
         if _submit:
             # submit button has unwrapped label and controls, different class
@@ -890,7 +890,7 @@ def formstyle_bootstrap3_stacked(form, fields):
                 e.add_class('form-control')
 
         if isinstance(label, LABEL):
-            label['_class'] += ' control-label'
+            label['_class'] = add_class(label.get('_class'),'control-label')
 
         parent.append(DIV(label, _controls, _class='form-group', _id=id))
     return parent
@@ -941,7 +941,7 @@ def formstyle_bootstrap3_inline_factory(col_label_size=3):
             elif controls is None or isinstance(controls, basestring):
                 _controls = P(controls, _class="form-control-static %s" % col_class)
             if isinstance(label, LABEL):
-                label['_class'] += ' control-label %s' % label_col_class
+                label['_class'] = add_class(label.get('_class'),'control-label %s' % label_col_class)
 
             parent.append(DIV(label, _controls, _class='form-group', _id=id))
         return parent