OriginTrackedPropertiesLoader drops the line following a comment that ends with a backslash

[zhoubiao188]

A comment line that ends with a backslash causes the property on the next line to be silently dropped. For example, this file

#\
key=value

loads no properties at all, whereas java.util.Properties loads key=value.

Cause

isNewDocument looks ahead through a comment to work out whether it is a --- document separator. The look-ahead uses CharacterReader#read(), which treats a trailing backslash as a line continuation and carries on reading on the following line. The reader therefore ends up past the end of the comment line and skipComment() discards the rest of the following line, losing the property.

Fix

The remainder of the comment line is now inspected with a new CharacterReader#readRaw() that does not process escape sequences, so the look-ahead can never cross the line boundary. This matches java.util.Properties, where a trailing backslash has no special meaning inside a comment.

Verification

• Added loadWhenCommentEndsWithBackslashLoadsFollowingProperties, loadWhenCommentWithExclamationPrefixEndsWithBackslashLoadsFollowingProperties and loadWhenDocumentSeparatorEndsWithBackslashLoadsSingleDocument. The first two fail without the change; the third one covers #---\, where b=b was previously swallowed as well.
• Differential test against java.util.Properties over 100000 generated small properties files: 98 inputs that previously disagreed now match, and no input that previously matched now disagrees.
• :core:spring-boot:checkFormat, checkstyleMain, checkstyleTest and the org.springframework.boot.env tests pass (43 tests in OriginTrackedPropertiesLoaderTests).