encode image_tag as utf8 in [x]html export

Should only affect Python 2

also removes redundant 'setDefaultButton' in Inline/External dialog,
which caused strange display of the two buttons.

IPython/frontend/qt/rich_text.py

@@ -11,6 +11,9 b' import re'
 # System library imports.
 from IPython.external.qt import QtGui
 
+# IPython imports
+from IPython.utils import py3compat
+
 #-----------------------------------------------------------------------------
 # Constants
 #-----------------------------------------------------------------------------
@@ -92,7 +95,6 b' class HtmlExporter(object):'
                     box.setInformativeText(info)
                     box.addButton(ib, QtGui.QMessageBox.NoRole)
                     box.addButton(eb, QtGui.QMessageBox.YesRole)
-                    box.setDefaultButton(ib)
                     layout.setSpacing(0)
                     layout.addWidget(box)
                     layout.addWidget(checkbox)
@@ -141,6 +143,8 b' def export_html(html, filename, image_tag = None, inline = True):'
     """
     if image_tag is None:
         image_tag = default_image_tag
+    else:
+        image_tag = ensure_utf8(image_tag)
 
     if inline:
         path = None
@@ -172,6 +176,8 b' def export_xhtml(html, filename, image_tag=None):'
     """
     if image_tag is None:
         image_tag = default_image_tag
+    else:
+        image_tag = ensure_utf8(image_tag)
 
     with open(filename, 'w') as f:
         # Hack to make xhtml header -- note that we are not doing any check for
@@ -210,6 +216,20 b' def default_image_tag(match, path = None, format = "png"):'
     return ''
 
 
+def ensure_utf8(image_tag):
+    """wrapper for ensuring image_tag returns utf8-encoded str on Python 2"""
+    if py3compat.PY3:
+        # nothing to do on Python 3
+        return image_tag
+    
+    def utf8_image_tag(*args, **kwargs):
+        s = image_tag(*args, **kwargs)
+        if isinstance(s, unicode):
+            s = s.encode('utf8')
+        return s
+    return utf8_image_tag
+
+
 def fix_html(html):
     """ Transforms a Qt-generated HTML string into a standards-compliant one.