Enhance the check for merging child line item into parent line item

[zliu1020-zz]

This bug has been reported from three different sources: Michael Wolz, One Rockwell and Pointer.

What it does and why:

Merge the child line item into the parent line item only when:

• Parent is present
• Child is present
• They share the same sku
• Child has a price of 0

Checklist:

• [x] Testing & QA: Passes tests and linting.
• [x] :tophat:: I have manually tested these changes.

Demo:

When a configurable line item and a simple line item with 0 price share the same sku, they are combined into one line item:

When two configurable line items share the same sku, they will not be merged:

In this example both have the same price

When two simple line items share the same sku, they will not be merged:

In this example they have different prices

When a configurable line item and a simple line item with positive price share the same sku, they will not be merged:

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Related issues:

Closes: https://github.com/Shopify/shopify_transporter/issues/123

[JordanVerasamy]

🎩 looks like this solves @michaelwolz's issue properly. (https://github.com/Shopify/shopify_transporter/issues/123)