Previously the following attributes in the meta tag:

```
<meta name="DC_IDENTIFIER" .../>
<meta name="EPRINTS.TITLE_" .../>
<meta property="OG:URL" .../>
<meta name="CITATION_DOI" .../>
<meta name="PRISM_TITLE" .../>
<meta name="TWITTER:SITE" .../>
```

produced a metadata dictionary like this:

```
{
  dc: {IDENTIFIER: [...] },
  eprints: {TITLE: [...]},
  facebook: {URL: [...]},
  highwire: {DOI: [...]},
  twitter: {SITE: [...]}
}
```

Now, with the modifications introduced in this PR, it produces a
metadata dictionary where all the keys are lowercase:
```
{
  dc: {identifier: [...] },
  eprints: {title: [...]},
  facebook: {url: [...]},
  highwire: {doi: [...]},
  twitter: {site: [...]}
}
```

I have confirmed that `h` expects the lowercase version of the keys
`identifier` and `doi`:

https://github.com/hypothesis/h/blob/73dacc722367c44903ff3c6f610712ba8f43934d/h/util/document_claims.py#L222

https://github.com/hypothesis/h/blob/73dacc722367c44903ff3c6f610712ba8f43934d/h/util/document_claims.py#L247

Hence, if they are not provided as lowercase version, they will be
ignored and the document equivalence will not work.