Add tests, and changelog entry about W293.

CHANGES.txt

@@ -5,6 +5,10 @@ Changelog
 0.5.1 (unreleased)
 ------------------
 
+* Blank lines with spaces yield W293 instead of W291: some developers
+  want to ignore this warning and indent the blank lines to paste their
+  code easily in the Python interpreter.
+
 * Fix E301: do not require a blank line before an indented block. (Issue #14)
 
 * Fix E203 to accept NumPy slice notation ``a[0, :]``. (Issue #13)

pep8.py

@@ -201,8 +201,10 @@ def trailing_whitespace(physical_line):
     physical_line = physical_line.rstrip('\x0c')  # chr(12), form feed, ^L
     stripped = physical_line.rstrip()
     if physical_line != stripped:
-        warning_code = "W291" if stripped != '' else "W293"
-        return len(stripped), "%s trailing whitespace" % warning_code
+        if stripped:
+            return len(stripped), "W291 trailing whitespace"
+        else:
+            return 0, "W293 blank line contains whitespace"
 
 
 def trailing_blank_lines(physical_line, lines, line_number):

testsuite/W291not.py

@@ -0,0 +1,3 @@
+class Foo(object):
+
+    bang = 12

testsuite/W293.py

@@ -0,0 +1,3 @@
+class Foo(object):
+
+    bang = 12