Make putc signatures type-preserving overloads

Per review feedback, type `StringIO#putc` / `Zlib::GzipWriter#putc` as
`(real) -> real | (String) -> String` instead of a flat
`(Numeric | String) -> (Numeric | String)`. `putc` returns its argument,
so the overloads preserve the caller's type; `real` is also more precise
than `Numeric` since `putc` does not accept `Complex`.

Author: zonuexe

stdlib/stringio/0/stringio.rbs

@@ -1271,7 +1271,8 @@ class StringIO
   # -->
   # See IO#putc.
   #
-  def putc: (Numeric | String arg0) -> (Numeric | String)
+  def putc: (real) -> real
+          | (String) -> String
 
   def puts: (*untyped arg0) -> nil
 

stdlib/zlib/0/gzip_writer.rbs

@@ -125,7 +125,8 @@ module Zlib
     # -->
     # Same as IO.
     #
-    def putc: (Numeric | String arg0) -> (Numeric | String)
+    def putc: (real) -> real
+            | (String) -> String
 
     # <!--
     #   rdoc-file=ext/zlib/zlib.c