(noticed this while investigating #1874)

Copy some code from FileManager::loadFile to reloadBuffer, to fix two bugs also when reloading file from disk. I think the bug 1 is quite important, as it affects "reload from disk" in a very common scenario (ANSI-only content, but force-detected as UTF-8 without BOM).

Bug 1:

1. The settings "New Document > UTF-8 + Apply to opened ANSI files" should be checked.
2. Create a file with only ANSI content (e.g. "aaa"). Save this file in ANSI. Close the file.
3. Open the file. It should be detected as UTF-8 (without BOM).
4. Execute menu command "Reload from Disk". The file is now detected as ANSI. Expected: should be detected as UTF-8.

Bug 2:

1. Create a file with this exact content:
   <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> ééé
2. Save it in ANSI and with a .html extension. Close the file.
3. Open the file. It should be detected as UTF-8 (and display garbage but that's "expected").
4. Execute command "Reload from Disk". The file is now detected as Windows-1255. Expected: should be detected as UTF-8.

By the way, maybe we should drop the getHtmlXmlEncoding method. For instance look at the HTML regexes, they are outdated (no html5), not flexible and in most cases wouldn't match. Also, I think this method does more harm than good. These <meta> tags don't determine the file encoding, but the expected file encoding.