File#path should return a new mutable String on every call

* Fixes #2115

Author: eregon

CHANGELOG.md

@@ -43,6 +43,7 @@ Compatibility:
 * Implement negative line numbers for eval (#1482).
 * Support refinements for `#to_s` called by string interpolation (#2110, @ssnickolay)
 * Module#using raises error in method scope (#2112, @ssnickolay)
+* `File#path` now returns a new mutable String on every call like MRI (#2115).
 
 Performance:
 

src/main/ruby/truffleruby/core/file.rb

@@ -108,8 +108,6 @@ module Constants
   PATH_SEPARATOR = ':'
   POSIX = Truffle::POSIX
 
-  attr_reader :path
-
   # The mode_t type is 2 bytes (ushort). Instead of getting whatever
   # value happens to be in the least significant 16 bits, just set
   # the value to 0 if it is greater than 0xffff. Also, negative values
@@ -1238,6 +1236,9 @@ def stat
     Stat.fstat Primitive.io_fd(self)
   end
 
+  def path
+    @path.dup
+  end
   alias_method :to_path, :path
 
   def truncate(length)