Mail transport encoding is difficult to set - Pry session

gistfile1.txt

[5] pry(main)> Mail::VERSION.version
=> "2.7.0"
[6] pry(main)> part = Mail::Part.new
=> #<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>>
[7] pry(main)> part.body = "Whatever"
=> "Whatever"
[8] pry(main)> puts part.encoded
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

Whatever
=> nil
[9] pry(main)> part.content_transfer_encoding = "quoted-printable"
=> "quoted-printable"
[10] pry(main)> part
=> #<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>, <Content-Transfer-Encoding: quoted-printable>>
[11] pry(main)> puts part.encoded
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

Whatever
=> nil
[12] pry(main)> part
=> #<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>
[13] pry(main)> part.transport_encoding = "quoted-printable"
=> "quoted-printable"
[14] pry(main)> part
=> #<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>
[16] pry(main)> puts part.encoded
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Whatever=
=> nil
[17] pry(main)> outer = Mail::Part.new
=> #<Mail::Part:70254001654700, Multipart: false, Headers: <Content-Type: text/plain>>
[18] pry(main)> outer.add_part part
=> [#<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>, <Content-Transfer-Encoding: quoted-printable>>]
[19] pry(main)> puts outer.encoded
Content-Type: text/plain;
 boundary="--==_mimepart_5ad492b173aaf_27603fe548842e5852040";
 charset=UTF-8
Content-Transfer-Encoding: 7bit


----==_mimepart_5ad492b173aaf_27603fe548842e5852040
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: 7bit

Whatever
----==_mimepart_5ad492b173aaf_27603fe548842e5852040--
=> nil
[20] pry(main)> part.body.encoding = "quoted-printable"
=> "quoted-printable"
[21] pry(main)> part
=> #<Mail::Part:70253998469580, Multipart: false, Headers: <Content-Type: text/plain>, <Content-Transfer-Encoding: 7bit>>
[22] pry(main)> part.body
=> #<Mail::Body:0x00007fca919aad68
 @ascii_only=true,
 @boundary=nil,
 @charset="US-ASCII",
 @encoding="quoted-printable",
 @epilogue=nil,
 @part_sort_order=["text/plain", "text/enriched", "text/html", "multipart/alternative"],
 @parts=[],
 @preamble=nil,
 @raw_source="Whatever">
[23] pry(main)> puts part.encoded
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Whatever=
=> nil
[24] pry(main)> puts outer.encoded
Content-Type: text/plain;
 boundary="--==_mimepart_5ad492b173aaf_27603fe548842e5852040";
 charset=UTF-8
Content-Transfer-Encoding: 7bit


----==_mimepart_5ad492b173aaf_27603fe548842e5852040
Content-Type: text/plain;
 charset=UTF-8
Content-Transfer-Encoding: quoted-printable

Whatever=

----==_mimepart_5ad492b173aaf_27603fe548842e5852040--
=> nil