Use proper order of 2 when generating RSA (#736)

with Java crypto providers generating primes with sizes that aren't 2^x
isn't really supported

Author: kares

spec/jwt/jwa/ps_spec.rb

@@ -48,7 +48,7 @@
     end
 
     context 'with a key length less than 2048 bits' do
-      let(:rsa_key) { OpenSSL::PKey::RSA.generate(2047) }
+      let(:rsa_key) { OpenSSL::PKey::RSA.generate(1536) }
 
       it 'raises an error' do
         expect do

spec/jwt/jwa/rsa_spec.rb

@@ -21,7 +21,7 @@
     end
 
     context 'with a key length less than 2048 bits' do
-      let(:rsa_key) { OpenSSL::PKey::RSA.generate(2047) }
+      let(:rsa_key) { OpenSSL::PKey::RSA.generate(1024) }
 
       it 'raises an error' do
         expect do