When bzip2 is used for compression, use gzip on the control.tar.

This fixes a bug where fpm would create an invalid debian package file.
When `--deb-compression bzip2` was used, fpm would create
'control.tar.bz2' file inside the debian package. Debian does not
support bzip2-compressed control files. Per the deb(5) manpage:

> The second required member is named control.tar. It is a tar archive containing the package control information, either not compressed (supported since dpkg 1.17.6), or compressed with gzip (with .gz extension) or xz (with .xz extension, supported since 1.17.6)

With this commit, when bzip2 is chosen for data compression, fpm will
use gzip compression on the control.tar file.

lib/fpm/package/deb.rb

@@ -612,7 +612,7 @@ def output(output_path)
         compression = "-z"
       when "bzip2"
         datatar = build_path("data.tar.bz2")
-        controltar = build_path("control.tar.bz2")
+        controltar = build_path("control.tar.gz")
         compression = "-j"
       when "xz"
         datatar = build_path("data.tar.xz")
@@ -863,12 +863,9 @@ def write_control_tarball
 
     # Tar up the staging_path into control.tar.{compression type}
     case self.attributes[:deb_compression]
-      when "gz", nil
+      when "gz", "bzip2", nil
         controltar = "control.tar.gz"
         compression = "-z"
-      when "bzip2"
-        controltar = "control.tar.bz2"
-        compression = "-j"
       when "xz"
         controltar = "control.tar.xz"
         compression = "-J"

spec/fpm/package/deb_spec.rb

@@ -520,9 +520,16 @@ def dpkg_field(field)
           deb.output(target)
         end
 
-        it "should use #{suffix} for data and control files" do
+        control_suffix = case flag
+          when 'bzip2'
+            'gz'
+          else
+            suffix
+        end
+
+        it "should use #{suffix} for data file and #{control_suffix} for control file" do
           list = `ar t #{target}`.split("\n")
-          insist { list }.include?("control.tar.#{suffix}")
+          insist { list }.include?("control.tar.#{control_suffix}")
           insist { list }.include?("data.tar.#{suffix}")
         end
       end