Lower casing the attribute names

[Vishwas]

Version Information

Python : sys.version_info(major=3, minor=7, micro=4, releaselevel='final', serial=0)
lxml.etree : (4, 4, 1, 0)
libxml used : (2, 9, 9)
libxml compiled : (2, 9, 9)
libxslt used : (1, 1, 33)
libxslt compiled : (1, 1, 33)

For HTML strings like

```html

<span myCamelCasedAttr="value">TestSpan</span>

```

parsing it with lxml changes the attribute name from camel case to lower case as

```html

<span mycamelcasedattr="value">TestSpan</span>

```

This especially is a problem if you are searching for attributes using string comparision. For example using BeautifulSoup it looks something like this.

```python

from bs4 import BeautifulSoup
import re

htmlstr = "<span mycamelcasedattr="value">TestSpan</span>"

soup = BeautifulSoup(htmlstr, "html.parser")
# Even if initialized as below.
# soup = BeautifulSoup(htmlstr, "lxml")
res = soup.find_all(True, {'myCamelCasedAttr': re.compile(r".*")})

# Returns
# res = []

```

Parsing HTML strings should maintain the casing of the attributes that gets parsed.

[Leonard Richardson (leonardr)]

This ticket was originally filed against Beautiful Soup: https://bugs.launchpad.net/beautifulsoup/+bug/1849211

The HTML spec defines tag names and attribute values as case-insensitive (http://w3c.github.io/html-reference/documents.html#case-insensitivity), so lxml's behavior is correct, but it might make sense to allow this as an option.