adjust tests for newer rspec version

spec/signature_spec.rb

@@ -24,38 +24,38 @@ module HMACAuth
 
         context 'valid' do
           let(:timestamp) { 10.minutes.ago.to_i.to_s }
-          it { should be_true }
+          it { should be_truthy }
         end
 
         context 'invalid' do
           let(:timestamp) { 20.minutes.ago.to_i }
-          it { should be_false }
+          it { should be_falsy }
         end
       end
     end
 
     describe '.sign' do
-      def signature(hash)
-        HMACAuth::Signature.sign(hash, secret: secret)['signature']
-      end
-
       describe 'hash' do
         subject { HMACAuth::Signature.sign(params, secret: secret) }
 
-        it { should be_a Hash }
-        its(['signature']) { should be_a String }
-        its(['timestamp']) { should be }
-        its(['b']) { should be_a String }
+        it { expect(subject).to be_a Hash }
+        it { expect(subject['signature']).to be_a String }
+        it { expect(subject['timestamp']).to be }
+        it { expect(subject['b']).to be_a String }
 
         context 'nested hash' do
           subject { HMACAuth::Signature.sign(params, secret: secret)['a'] }
-          it { should be_a Hash }
-          its(['d']) { should == '4' }
-          its(['c']) { should == '3' }
+          it { expect(subject).to be_a Hash }
+          it { expect(subject['d']).to eq '4' }
+          it { expect(subject['c']).to eq '3' }
         end
       end
 
       describe 'unsorted input' do
+        def signature(hash)
+          HMACAuth::Signature.sign(hash, secret: secret)['signature']
+        end
+
         let(:hasha) { { a: 1, b: { c: 3, d: 4 } } }
         let(:hashd) { { b: { d: 4, c: 3 }, a: 1 } }