Add page tests

[johnfercher]

Description

• Add page unit tests

Related Issue

Checklist

check with "x", if applied to your change

• [ ] All methods associated with structs has func (<first letter of struct> *struct) method() {} name style.
• [x] Wrote unit tests for new/changed features.
• [x] Followed the unit test when,should naming pattern.
• [ ] Updated docs/doc.go and docs/*
• [ ] Updated pkg/pdf/example_test.go
• [ ] Updated README.md
• [ ] Executed make examples to update all examples inside docs/examples.
• [ ] New public methods/structs/interfaces has comments upside them explaining they responsibilities
• [x] Executed make dod with none issues pointed out by golangci-lint

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (21b2202) 79.84% compared to head (054319e) 81.57%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #391 +/- ## ========================================== + Coverage 79.84% 81.57% +1.74% ========================================== Files 61 61 Lines 2018 2018 ========================================== + Hits 1611 1646 +35 + Misses 379 345 -34 + Partials 28 27 -1 ``` | [Flag](https://app.codecov.io/gh/johnfercher/maroto/pull/391/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/johnfercher/maroto/pull/391/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher) | `81.57% <ø> (+1.74%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=John+Fercher#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.