org-colview: Store properties in upper case

org-colview: Store properties in upper case

* lisp/org-colview.el (org-columns-compile-format): Property is
  upper-cased.  Title is not, however.
(org-columns--displayed-value):
(org-columns--collect-values):
(org-columns--autowidth-alist):
(org-columns--overlay-text):
(org-columns--display-here):
(org-columns--display-here-title):
(org-columns-next-allowed-value):
(org-columns):
(org-columns-widen):
(org-columns-update):
(org-columns--capture-view):
(org-dblock-write:columnview):
(org-agenda-colview-summarize): Since properties in compiled format are
upper-cased, remove the `upcase' or `assoc-string' dance.

* testing/lisp/test-org-colview.el (test-org-colview/columns-new):
(test-org-colview/columns-update): Add case-sensitivity tests.

`assoc-string' is still necessary in functions where property is
provided by the user, e.g. `org-columns-update'.