fix: Explicitly mark buffer variable as unfrozen (#59)

In a future version of Ruby, literal strings will be frozen by default.
To avoid this causing an issue (and raising deprecation warnings now),
we are explicitly marking this literal string as unfrozen.

Author: keelerm84

lib/ld-eventsource/impl/buffered_line_reader.rb

@@ -1,4 +1,3 @@
-
 module SSE
   module Impl
     class BufferedLineReader
@@ -16,7 +15,7 @@ class BufferedLineReader
       # @return [Enumerator]  an enumerator that will yield one line at a time in UTF-8
       #
       def self.lines_from(chunks)
-        buffer = "".b
+        buffer = +"".b
         position = 0
         line_start = 0
         last_char_was_cr = false