Use run_cell for %paste, instead of plain exec.

Closes gh-865, but introduces test failures for %cpaste

IPython/core/magic.py

@@ -3246,7 +3246,7 b' Defaulting color scheme to \'NoColor\'"""'
         if not par:
             b = textwrap.dedent(block)
             self.user_ns['pasted_block'] = b
-            exec b in self.user_ns
+            self.run_cell(b)
         else:
             self.user_ns[par] = SList(block.splitlines())
             print "Block assigned to '%s'" % par

IPython/core/tests/test_magic.py

@@ -316,6 +316,7 b' def check_cpaste(code, should_fail=False):'
     _ip.user_ns['code_ran'] = False
 
     src = StringIO()
+    src.encoding = None   # IPython expects stdin to have an encoding attribute
     src.write('\n')
     src.write(code)
     src.write('\n--\n')