doRenderTags() now handles a top-level tagFunction()

[cpsievert]

Closes #203

[wch]

I'm a bit concerned that if tag functions are passed directly to doRenderTags(), information like html dependencies will be lost. In my reading of the code is that, until this change, doRenderTags() has been meant to be used with plain tag-like objects which have already had that processing done (usually by renderTags()). After this change, it's unclear to me exactly what doRenderTags() is supposed to accept -- it can now accept tagFunctions, but in so doing it will lose html dependency info.

When does this change actually makes a difference in behavior, and does it make more sense to fix those cases?

[cpsievert]

Good point.

The motivation for this change is to make shiny:::processDeps(buildTabItem(...)) (i.e., insertTab() et. al) work in rstudio/shiny#3315.

I'll revert this change and add the special case in processDeps() instead