mail gem breaks attachment file with encoding Quoted-Printable.

I found this issue when I encode mail with binary file attachment in Quoted-Printable.

This is sample case.
```ruby
require 'mail'

# 1. Create Mail with Quoted-Printable
mail = Mail.new
part = Mail::Part.new
bin = File.binread("./some_binary.jpg")
part.body = [bin].pack("M")
part.content_transfer_encoding = "quoted-printable"
part.transport_encoding = Mail::Encodings::QuotedPrintable
mail.parts << part
File.binwrite("./tmp/original.eml", mail.to_s)

# 2. Read mail and write

mail = Mail.read("./tmp/original.eml")
File.binwrite("./tmp/after.eml", mail.to_s)
```

This is difference between original.eml and after.eml.

```diff
diff --git 1/tmp/original.eml 2/tmp/after.eml
index 7d2c038..506176e 100644
--- 1/tmp/original.eml
+++ 2/tmp/after.eml
@@ -27,15 +27,16 @@ Content-Transfer-Encoding: quoted-printable
=01=01=00=02=03=00=00=00=00=00=01=02=11=03=12!1=04A=05=13Q"a2q=15=81=B1=FF=
=DA=00=0C=03=01=00=02=11=03=11=00?=00=1C|f=98=08=A2=C6i@=E2=BFW?9=07=B2A=14=
=80=F7=A2=04=E4=FB=D2 =13=C5=02=07=B7=A5"=93=9A =1C=D2=8E=B4=0D=02=03=9E=94=
-=FBD=CCQ=12=93=CD8=14=08=08H=02=9Bl=8C=1A6=DC=F1L=13=8A=00=1E=DEim=10{
+=FBD=CCQ=12=93=CD8=14=08=08H=02=9Bl=8C=1A6=DC=F1L=13=8A=00=1E=DEim=10{=0D=
+
,sM=1C=98=A4=00=80=A5=B6=8B=B6=96=DA=07=E8=06=DAr=98=E9E=00=FBR=DB=88=A0@=
=B6=E3=8C=D3G?=D6=8B=B78=A4=A0&=80=05=1C=D3m=A2m=A5=B6=80=07=B4SE=16&i=14=
=FD(=001=DA=91O=D6=8AS=8E=0D4`=E6=90=D2=03=B6:R=83=EDE=8C=D3m=89=14=C0=19=
-Gz=8CGJ)=039=A6=89=C4=D0=82=81=ED=00qM=00=0E1E =C6M4b=90P";
+Gz=8CGJ)=039=A6=89=C4=D0=82=81=ED=00qM=00=0E1E =C6M4b=90P";=0D
b=9Fl=D1H=3D)=B6=CF\=F5=A0(=14w=A6=DBE"=04=E6=A2S@=03=81=C7=EBM=B4DQ=00=C4=
R=8A=07@=8AzTv=E4=D1J}=A9=A0=CD=00=08=A4v8=A8=C7<=D1=A2=A2S#=8A=01=A0[y=A8=
=C1=E2=8D=B7=EBLA=C9=14=82=80=ED D=7F=D8=D4v=FE=94h"=98=8Fj`=90=18=A6#"=8D=
-=1E=D5=10=07JV:=04=13Q=8C=9A)=07=E1M=04=0CM =02=A4=8C=FB=D4
+=1E=D5=10=07JV:=04=13Q=8C=9A)=07=E1M=04=0CM =02=A4=8C=FB=D4=0D
z=8A6=DE=D4=D1=88=E9@#=AA =81=EFNR>=B4M=A7=A5 =9CI=AB%=03=DA =E2=96=DFj$=01=
Kn S=00a<=C6iG1=D6=88=13=8CR=03=BD =06=13=CC=D2=DB=EDD#=9A@c=A5=00=0C=8AQ=
```

This problem cause from ruby Array#pack("M") behavior.
Ruby's Array#pack("M") doesn't support Quoted-Printable with binary, so ruby does not encode "\n".

```
["\n"].pack("M") # => "\n"
```

and Mail library converts "\n" to "\r\n" in to_crlf. so break binary file.
I found similar issue in #1010

I asked to ruby community. (https://bugs.ruby-lang.org/issues/14352 sorry this discussion in Japanese.) and I got a answer which this behavior should not be change.

so I propose to change implementation in Mail gem's Quoted-Printable encoder as follow.

```diff
diff --git i/lib/mail/encodings/quoted_printable.rb w/lib/mail/encodings/quoted_printable.rb
index 193778c..6a04833 100644
--- i/lib/mail/encodings/quoted_printable.rb
+++ w/lib/mail/encodings/quoted_printable.rb
@@ -16,11 +16,18 @@ module Mail
# Decode the string from Quoted-Printable. Cope with hard line breaks
# that were incorrectly encoded as hex instead of literal CRLF.
def self.decode(str)
- str.gsub(/(?:=0D=0A|=0D|=0A)\r\n/, "\r\n").unpack("M*").first
+ str.unpack("M").first
end

def self.encode(str)
- [str].pack("M")
+ [str].pack("M").gsub(/(=0D\n|(?

Start in lib/mail/encodings/quoted_printable.rb and reproduce the binary-attachment round trip using the Ruby Array#pack("M") behavior shown in the issue. Review the quoted-printable encoding tests, including the text case that currently fails with the proposed change; done means preserving binary attachments while making the intended compatibility behavior explicit.

Written by the indexing model from the issue text.