Add specs for coercion for FFI::Struct and FFI::AbstractMemory

* This is generally handled in FFI::Pointer #write/#put methods.

Author: eregon

spec/ffi/struct_spec.rb

@@ -326,6 +326,17 @@ def test_num_field(type, v)
     expect(s.pointer.send("get_#{type.to_s}", 0)).to eq(v)
     s.pointer.send("put_#{type.to_s}", 0, 0)
     expect(s[:v]).to eq(0)
+
+    # Test coercion
+    obj = double("coerce")
+    expect(obj).to receive(:to_int).and_return(v)
+    s[:v] = obj
+    expect(s.pointer.send("get_#{type.to_s}", 0)).to eq(v)
+
+    zero = double("zero")
+    expect(zero).to receive(:to_int).and_return(0)
+    s.pointer.send("put_#{type.to_s}", 0, zero)
+    expect(s[:v]).to eq(0)
   end
   def self.int_field_test(type, values)
     values.each do |v|
@@ -358,6 +369,15 @@ def self.int_field_test(type, values)
     value = 1.23456
     s[:v] = value
     expect((s.pointer.get_float(0) - value).abs).to be < 0.0001
+
+    # Test coercion
+    obj = double("coerce")
+    expect(obj).to receive(:to_f).and_return(42.0)
+    s[:v] = obj
+    expect((s.pointer.get_float(0) - 42.0).abs).to be < 0.0001
+
+    s.pointer.put_float(0, 1.0)
+    expect(s.pointer.get_float(0)).to eq(1.0)
   end
 
   it ":double field r/w" do
@@ -368,6 +388,15 @@ def self.int_field_test(type, values)
     value = 1.23456
     s[:v] = value
     expect((s.pointer.get_double(0) - value).abs).to be < 0.0001
+
+    # Test coercion
+    obj = double("coerce")
+    expect(obj).to receive(:to_f).and_return(42.0)
+    s[:v] = obj
+    expect((s.pointer.get_double(0) - 42.0).abs).to be < 0.0001
+
+    s.pointer.put_double(0, 1.0)
+    expect(s.pointer.get_double(0)).to eq(1.0)
   end
   module EnumFields
     extend FFI::Library