Add more tests for Julian Date conversion (#122)

This adds more variety to the test of Julian Date conversion, based on
examples from Astronomical Algorithms book.

spec/astronoby/epoch_spec.rb

@@ -48,6 +48,33 @@
 
       expect(described_class.from_time(time)).to eq(2457153.25)
     end
+
+    # Source:
+    #  Title: Astronomical Algorithms
+    #  Author: Jean Meeus
+    #  Edition: 2nd edition
+    #  Chapter: 7 - Julian Day
+    [
+      [Time.utc(2000, 1, 1, 12), 2451545],
+      [Date.new(1999, 1, 1), 2451179.5],
+      [DateTime.new(1987, 6, 19, 12), 2446966],
+      [Date.new(1988, 1, 27), 2447187.5],
+      [Time.utc(1988, 6, 19, 12), 2447332],
+      [Date.new(1900, 1, 1), 2415020.5],
+      [Date.new(1600, 1, 1), 2305447.5],
+      [Date.new(1600, 12, 31), 2305812.5],
+      [Date.new(837, 4, 10), 2026871.5],
+      [Date.new(-123, 12, 31), 1676496.5],
+      [Date.new(-122, 1, 1), 1676497.5],
+      [DateTime.new(-1000, 7, 12, 12), 1356001],
+      [Date.new(-1000, 2, 29), 1355866.5],
+      [DateTime.new(-1001, 8, 17, 21, 36), 1355671.4],
+      [DateTime.new(-4712, 1, 1, 12), 0]
+    ].each do |time_or_date, expected|
+      it "returns the Julian day number associated with the time" do
+        expect(described_class.from_time(time_or_date)).to eq(expected)
+      end
+    end
   end
 
   describe "::to_utc" do
@@ -107,5 +134,25 @@
 
       expect(time).to eq Time.utc(2012, 4, 1, 20, 52, 48)
     end
+
+    # Source:
+    #  Title: Astronomical Algorithms
+    #  Author: Jean Meeus
+    #  Edition: 2nd edition
+    #  Chapter: 7 - Julian Day
+    [
+      [Time.utc(2000, 1, 1, 12), 2451545],
+      [Time.utc(1999, 1, 1), 2451179.5],
+      [Time.utc(1987, 6, 19, 12), 2446966],
+      [Time.utc(1988, 1, 27), 2447187.5],
+      [Time.utc(1988, 6, 19, 12), 2447332],
+      [Time.utc(1900, 1, 1), 2415020.5],
+      [Time.utc(1600, 1, 1), 2305447.5],
+      [Time.utc(1600, 12, 31), 2305812.5]
+    ].each do |expected_time, epoch|
+      it "returns the Julian day number associated with the time" do
+        expect(described_class.to_utc(epoch)).to eq(expected_time)
+      end
+    end
   end
 end