Support ruby-2.4

[azuchi]

OpenSSL::PKey::EC in openssl of Ruby 2.4 contains bugs that are not in before 2.3.

If set public key using OpenSSL::PKey::EC#public_key= then set EC group encoding to :compressed, and get the public key again using OpenSSL::PKey::EC#public_key, so public key encoding is :uncompressed.

key = OpenSSL::PKey::EC.new("secp256k1")
pub_key = '03cdd34ec0a05d91c026fe8cb74434923075d3acc20f3f673fb855c8f2c04ca522' # compressed pubkey

# success case
point = OpenSSL::PKey::EC::Point.new(key.group, OpenSSL::BN.new(pub_key, 16))
point.group.point_conversion_form = :compressed

# with 2.3 and 2.4, following result is :compressed
puts point.group.point_conversion_form

# error case
key.public_key = OpenSSL::PKey::EC::Point.new(key.group, OpenSSL::BN.new(pub_key, 16))
key.public_key.group.point_conversion_form = :compressed

# with 2.4, following results is :uncompressed
puts key.public_key.group.point_conversion_form

https://bugs.ruby-lang.org/issues/13100

Bitcoin::Key#pub_compressed is affected by this and will not work properly with Ruby 2.4. Therefore, I added work-around patch to work with Ruby 2.4.