Fix partial line protection issue

Fix partial line protection issue

* lisp/org-html.el (org-export-as-html): Do not treat partially
protected lines as if they were fully protected.

Nicolas Goaziou writes:

> Here is a problem when a latex fragment is split across two lines and
> an emphasize follows. The text won't be italicized upon exporting to
> HTML.
>
> =====
> * latex-fragments bug
>
>  Imagine we have a formula starting here $e^{i\pi} +
>  1 = 0$. Now we have a problem with /emphasize/.
> =====
>
> This is because the line starts with a char with 'org-protected
> property and, thus, get caught by the "Protected HTML" (org-html.el
> l. 1216) part of `org-export-as-html'. In others words, the line is
> inserted as-is in the output buffer, before getting any
> transformation.
>
> I'm not sure how it should be done (I don't get yet the usefulness of
> this "Protected HTML" part), but that piece of code may be moved after
> the `org-html-expand' call, as long as every sub-function in
> `org-html-expand' has a check to prevent modifying protected stuff
> (this not yet the case for `org-export-with-emphasize' and
> `org-html-protect' while others seem ok).
>
> But even in this case, every function getting called after that would
> be ignored. So, for example, links would not be inserted.
>
> Couldn't the "Protected HTML" part be removed altogether?