fixed org-ehtml-format-headline

[jnear]

1. org-ehtml-format-headline-function had a typo (headine vs headline)
2. org-ehtml-format-headline-function can't handle a nil headline (I'm not sure why this can happen, but it does on some of my org files)
3. org-html-headline looks at the plist on info to find the html-format-headline-function, so let-binding org-html-format-headline-function doesn't work (maybe this has changed recently?)