- dedenter.rb: Fix squiggly heredoc line continuation handling (#819)

Firstly, this fixes an issue with non-interpolating squiggly heredocs, where line continuation characters should be ignored. Two things were broken:

- The line continuation backslash would be removed, as well as the subsequent carriage return
- The subsequent line would not be dedented.

For example, the following source:

```ruby
<<~'END'
  foo \
  bar
END
```

Would be parsed as follows:

```
(dstr
  (str "foo ")
  (str "  bar\n"))
```

With the change, the result is:

```
(dstr
  (str "foo \\\n")
  (str "bar\n"))
```

This also fixes an issue with interpolating squiggly heredocs, where an escaped backslash followed by a newline would be interpreted as a line continuation. This is issue #789.

In that case, the following source:

```ruby
<<~END
  foo \\
  bar
END
```

Would (again) be parsed as follows:

```
(dstr
  (str "foo ")
  (str "  bar\n"))
```

With the change, the result is:

```
(dstr
  (str "foo \\\n")
  (str "bar\n"))
```

lib/parser/lexer/dedenter.rb

@@ -38,7 +38,13 @@ def dedent(string)
       # Prevent the following error when processing binary encoded source.
       # "\xC0".split # => ArgumentError (invalid byte sequence in UTF-8)
       lines = string.force_encoding(Encoding::BINARY).split("\\\n")
-      lines.map! {|s| s.force_encoding(original_encoding) }
+      if lines.length == 1
+        # If the line continuation sequence was found but there is no second
+        # line, it was not really a line continuation and must be ignored.
+        lines = [string]
+      else
+        lines.map! {|s| s.force_encoding(original_encoding) }
+      end
 
       if @at_line_begin
         lines_to_dedent = lines

test/test_parser.rb

@@ -439,6 +439,22 @@ def test_parser_bug_640
       SINCE_2_3)
   end
 
+  def test_dedenting_non_interpolating_heredoc_line_continuation
+    assert_parses(
+      s(:dstr, s(:str, "baz\\\n"), s(:str, "qux\n")),
+      %Q{<<~'FOO'\n  baz\\\n  qux\nFOO},
+      %q{},
+      SINCE_2_3)
+  end
+
+  def test_dedenting_interpolating_heredoc_fake_line_continuation
+    assert_parses(
+      s(:dstr, s(:str, "baz\\\n"), s(:str, "qux\n")),
+      %Q{<<~'FOO'\n  baz\\\\\n  qux\nFOO},
+      %q{},
+      SINCE_2_3)
+  end
+
   # Symbols
 
   def test_symbol_plain