Fix ``%history -f foo`` in kernel.

Closes gh-4582

IPython/kernel/zmq/ipkernel.py

@@ -341,7 +341,7 b' class Kernel(Configurable):'
         
         try:
             content = parent[u'content']
-            code = content[u'code']
+            code = py3compat.cast_unicode_py2(content[u'code'])
             silent = content[u'silent']
             store_history = content.get(u'store_history', not silent)
         except:

IPython/utils/py3compat.py

@@ -83,6 +83,7 b' if sys.version_info[0] >= 3:'
     str_to_bytes = encode
     bytes_to_str = decode
     cast_bytes_py2 = no_code
+    cast_unicode_py2 = no_code
     
     string_types = (str,)
     unicode_type = str
@@ -139,6 +140,7 b' else:'
     str_to_bytes = no_code
     bytes_to_str = no_code
     cast_bytes_py2 = cast_bytes
+    cast_unicode_py2 = cast_unicode
     
     string_types = (str, unicode)
     unicode_type = unicode