Keep uppercase for tocify's hash (fix #975)

[atusy]

This PR fixes #975

For floating TOC, custom hashGenerator is defined by 5b98ee1 .
There, .toLower() is introduced, which made "Foo" and "foo" indistinctive.
I fixed the problem by removing .toLower().

reprex

---
title: title
output:
  html_document:
    toc: true
    toc_float: true
---

# foo

```{r}
iris
```

# Foo

[yihui]

Sounds good. Thank you!